1. Field of the Invention
The present invention relates to a method of producing a thin film transistor device and, more specifically, to a method of producing a plurality of thin film transistors on a substrate of glass or the like.
2. Description of the Relates Art
In producing a large area device that includes a number of thin film transistors, there has been performed a process in which a number of thin film transistors are formed on a substrate of glass or the like, and then the substrate is cut off at predetermined sizes into final products.
FIG. 5 shows a conventional method of producing a thin film transistor device. In FIG. 5, a substrate 1 of glass or the like, has formed thereon a thin film transistor 2, a pad 3, a substrate cutting line 4, a wiring pattern 8, a gate G, a source S, and a drain D.
On the substrate 1, the thin film transistors 2 are formed by stacking layers and, at the same time, the wiring patterns 8, and the pads 3 are formed so as to be connected with the electrodes of the respective thin film transistors 2. Thereafter, the substrate 1 is cut along the substrate cutting line 4 so as to obtain a thin film transistor device.
In such a conventional method of producing a thin film transistor device as mentioned above, however, there has been a problem in that the insulation between the gate and drain or between the gate and source may be broken by discharge due to static electricity produced during the manufacturing process.
FIG. 6 is a sectional view of stacked layers of a conventional thin film transistor. In FIG. 6 a thin film transistor 2, includes a wiring pattern 8, a gate 9, an insulating layer 10, a semiconductor active layer 11, an ohmic contact layer 12, a source 13, a drain 14, and a conductor layer 15. The gate 9 is elongated in the direction perpendicular to the paper and is connected to the gates of other thin film transistors (with reference to G in FIG. 5).
The respective electrodes of the source, drain, and gate are formed of electric conductors, and the insulating layer 10 is interposed between the electrodes. Accordingly, there exists electrostatic capacity between the gate and source or between the gate and drain.
FIG. 7 is a diagram showing the existence of electrostatic capacity between electrodes of a thin film transistor. An electrostatic capacity C.sub.1 exists between the gate G and source S, and an electrostatic capacity C.sub.2 exists between the gate G and drain D.
During the production process, undesirable static electricity sometimes forms while the substrate 1, having thin film transistors 2 formed thereon, is being conveyed through conveyor lines.
The formation of static electricity may take place, for example, when the substrate 1 moves from one conveyor line to another. If there is a difference in speed between the conveyor lines, one of the conveyor lines may slip on the substrate 1 causing the substrate 1 to be rubbed by the slipping conveyor line. This rubbing between the substrate 1 and the slipping conveyor line causes friction between the two and results in the formation of static electricity.
The marks "+" in FIG. 6 represent static electricity. Although there is no significant problem in the case where the static electricity is distributed uniformly, there is a problem if the static electricity is locally concentrated. However, static electricity is apt to be concentrated locally (because a substrate has various layers formed thereon in various shapes). As a result of such local static charge concentration, the potential difference between the gate and drain becomes large and the insulation between the gate and drain becomes large and the insulation between the gate and drain is subjected to discharge-breakdown. The same applies to the insulation between the gate and source. If such discharge-breakdown occurs, the thin film transistor can no longer perform its function.